1. Field of the Invention
The present invention relates to microstrip antennas and, in particular, to a microstrip antenna that is well suited for use in mobile radio applications.
2. Description of the Related Art
The typical microstrip antenna includes a ground plane and a microstrip element that are located parallel to one another and between which is located a dielectric material. Also included in the typical microstrip antenna is a transmission line that provides a communication path for radio frequency (rf) signals to and from the microstrip element and the ground plane. To transmit rf signals using the microstrip antenna, an rf signal is applied by a transmitter to the transmission line which, in turn, applies the rf signal to the microstrip element and the ground plane. In response, an electromagnetic signal is radiated between the edges of the microstrip element and the ground plane, in a pattern and at a frequency that is dependent upon, among other things, the positional and dimensional characteristics of the microstrip element, the ground plane, and the dielectric. Conversely, during reception, the microstrip element and the ground plane resonate upon interacting with an electromagnetic signal of an appropriate frequency to produce an rf signal that is provided to by the transmission line to a receiver for decoding.
Microstrip antennas have been found to be particularly well-suited to mobile radio communications and the subclass of portable radio communications, due, at least in part, to their substantially omnidirectional radiation patterns, i.e., radiation patterns that exhibit substantially the same gain in any direction within a particular plane of interest (generally a horizontal plane), and due to the relatively high efficiency that this type of antenna is capable of achieving in combination with its relatively small size and weight. A substantially omnidirectional radiation pattern is of fundamental concern in mobile radio communications because of the continually changing orientation of the mobile radio with respect to the radio with which communications are being conducted, hereinafter referred to as the communicating radio. For example, in cellular radio networks, the orientation of the mobile radio that is located in an automobile or other mobile vehicle changes with respect to the communicating radio as the location of the automobile changes within the cell, i.e., the area within which the communicating radio is operational. As a consequence, it is important that the radiation pattern of the antenna be substantially omnidirectional. Similarly, a high efficiency is of concern in mobile radio communications because the distance between the mobile radio and the communicating radio typically varies widely. Given this variation, an antenna with a high efficiency allows communications to be conducted over a correspondingly broad range of distances between the mobile radio and the communicating radio.
Among the factors that can adversely affect the radiation pattern and/or the gain of a microstrip antenna is the manner in which the transmission line is connected to the microstrip element and/or the ground plane. For example, U.S. Pat. No. 4,700,194 ('194), which issued on Oct. 13, 1987 to Ogawa et al., and is entitled "Small Antenna," indicates that the location of the connection between the transmission line and the ground plane has a substantial effect on the radiation pattern and gain of the microstrip antenna.
Another feature of the connection between the transmission line and the microstrip element that can adversely affect the radiation pattern and gain of the microstrip antenna is the inductance associated with the connection. For example, when a coaxial cable is used for the transmission line, a length of the center conductor of the coaxial cable must be exposed, i.e., extend beyond the end of the outer conductor, for connection to the microstrip element. The more of the center conductor that is exposed, the greater the resulting inductance. As the inductance increases, the mismatch in impedance between the coaxial cable and the microstrip element increases. This, in turn, adversely affects the radiation pattern and gain of the microstrip antenna.
U.S. Pat. No. 4,835,541 ('541), which issued on May 30, 1989 to Johnson et al. and is entitled "Near-Isotropic Low-Profile Microstrip Radiator Specially Suited for Use as a Mobile Vehicle Antenna," proposes the use of an impedance matching network to counteract the inductance associated with the connection of the transmission line to the microstrip element. The proposed impedance matching network, while possibly addressing the performance drawbacks associated with an impedance mismatch, reduces the desirability of the resulting microstrip antenna for mobile radio communication applications. Namely, the impedance matching network proposed in the U.S. Pat. No. '541 adds several additional parts to the microstrip antenna that must be connected to one another during manufacture. Since a characteristic of most, if not all, mobile radio communication applications is that the antenna is subjected to a considerable amount of physical stress, such as vibrations and temperature fluctuations, the corresponding increase in the number of interconnections necessitated by the increased number of parts associated with the impedance matching network make the resulting microstrip antenna susceptible to failure.
Another requirement or highly desirable feature in many mobile radio communication applications is that the antenna be concealed from view. For example, it is desirable to conceal the antenna associated with the cellular telephone in an automobile so that thieves are not readily able to determine whether or not the automobile contains a cellular phone. The U.S. Pat. No. '541 discloses a microstrip antenna that is concealed by mounting it in the space between a plastic roof and a headliner in a passenger vehicle. Use, however, of the embodiment of the microstrip antenna that employs an impedance matching network increases the overall height profile of the antenna and, as a consequence, reduces the ability of such an antenna to be concealed. Moreover, the impedance matching network necessitates significant reworking of the manner in which the microstrip antenna is mounted to the roof of the automobile because the impedance matching network makes impossible the flush mounting of the microstrip antenna to the roof that is possible when the impedance matching network is omitted.
Also of concern in many mobile radio communication applications is the relationship between the number of discrete parts comprising the microstrip antenna and the cost of assembling the antenna. Specifically, as the number of discrete parts comprising the microstrip antenna increases, the cost of the microstrip antenna increases due to the increased amount of time necessary to assemble the parts into an antenna. This increased cost, in turn, inhibits the use of microstrip antennas in, for example, mass consumer market applications, such as the cellular telephone market, even though the microstrip antenna possesses performance and/or structural advantages over alternative types of antennas.
Also desirable in many mobile radio communication applications is the ability to readily attach and detach an antenna from a surface. For example, if it is not feasible to conceal the antenna, then the ability to attach the antenna to an exposed surface when the antenna is in use and detach the antenna when not in use is, in many instances, a highly desirable feature.
Yet of further concern in portable or mobile communications by radio is the exterior aspect of the antenna. For example, if the antenna is used in an application where it is exposed to external forces, such as wind, the external aspect of the antenna can affect the ability of the antenna to withstand such forces. Moreover, in many consumer oriented mobile radio applications, such as cellular telephones, the exterior aspect of the antenna typically has significant impact on the appeal of the antenna to the consumer.
Based on the foregoing, there is a need for a microstrip antenna that addresses the deficiencies of known microstrip antennas and, in particular, of those microstrip antennas that are employed in mobile radio communication applications. Specifically, there is a need for a microstrip antenna that provides an improved degree of reliability, that is readily adapted to concealment, and that employs a low part count to realize part as well as manufacturing cost benefits. In this regard, there is a need for a microstrip antenna that substantially eliminates the use of an impedance matching network. In addition, a microstrip antenna is needed that provides a substantially omnidirectional radiation pattern and a high efficiency. Further, a microstrip antenna that can be readily attached and detached from a surface is needed. Moreover, there is a need for a microstrip antenna with an external aspect that addresses the external forces that can affect the operation of the antenna and/or the appeal of the antenna to the consumer.